1. Field of the Invention
This invention relates to a method of shot peening coil springs.
2. Description of the Background of the Invention
Shot peening is a well known and widely accepted technique for increasing strength of metal components. Coil springs are often repeatedly stressed during use and benefit from shot peening. Accordingly, it is desirable to shot peen springs to increase strength. Tests have shown that, while failure can occur anywhere on the spring, failures are more common on the inside of the spring making it desirable to peen both the inner and outer diameters of coiled springs. Most compression springs are normally “open” in there relaxed state so that there is a gap between the coils that is large enough to permit peening by conventional methods. Accordingly, peening of the inner diameter of a compression coil spring may be easily accomplished, except for compression springs having very small gaps between the coils that prevent peening of the inner diameter of the spring by conventional methods. Coils of extension springs normally engage one another, permitting peening only on the outer diameter unless the spring is stretched to open passages between the coils. This is not difficult for relatively short springs, which may be stretched within the peening cabinet, but it limits the length of springs that can be peened to the size of the peening cabinet. Other prior art methods for peening the inner diameter of coil springs include inserting a lance into the spring, but again, only relatively short springs may be peened in this manner. In all cases, since the size of peening cabinets is limited, the length of extension springs that may be peened is also limited.